add support for generic open ai assistant api

We should look into bringing open ai assistant api that works with any LLMs.

Reference:
* https://github.com/metaskills/experts and https://news.ycombinator.com/item?id=40395107
* https://github.com/supercorp-ai/supercompat
* https://platform.openai.com/docs/assistants/overview

Review the provided references to understand the OpenAI Assistants API and existing compatibility layers. Examine the langchain-rust codebase for existing LLM integrations and API client patterns. Identify where to add a new module for assistant API support, likely in a new crate or under an `assistants` module. Determine the core structs and traits needed to mirror the OpenAI interface while remaining generic.

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